1. Field of the Invention
The present invention relates to refrigerant systems, and more particularly to heat pump refrigerant systems equipped with supplemental heating.
2. Description of the Related Art
Some heat pump systems are equipped with supplemental gas heating means. During operation of these systems, when the ambient temperature falls below a certain level (at specified indoor conditions), it becomes more efficient to switch from utilizing electric energy and running a heat pump in a heating mode to heating an indoor environment by engaging supplemental gas (natural gas, propane, butane, etc.) heat, supplemental heat from other commodities such as oil, heated water, and/or heated air. Currently this is accomplished by setting a thermostat within a conditioned (heated in this case) environment that switches between the heat pump mode of operation, and gas heating at a predetermined ambient temperature, normally at about 20° F., for a conventional indoor temperature range about 70° F.+/−5° F. Sometimes an installer or a consumer can also adjust the standard factory setting by removing a panel to get access to this internal thermostat. When the ambient temperature falls below this predetermined setting, the gas or other supplemental heat is engaged.
A major drawback of having the predetermined setting of switching between the heat pump mode of operation and the supplemental heating is that this setting cannot be changed in real time, as it corresponds to a certain predetermined value that is established either at the factory or by the user, i.e., installer. The value of this predetermined setting is based on rule-of-thumb knowledge regarding heat pump system operation, thermal behavior of the heated structure, as well as electricity and gas (or other commodity) prices.
Even worse, for a vast majority of prior art cases, and especially for single-circuit heat pump systems, a thermostat setting was typically based on a balance point of a structure that would depend on indoor and outdoor conditions (primarily temperatures for heating systems). A balance point is an outdoor temperature at which a heat pump does not provide enough heat for a specific thermostat setting corresponding to a specific indoor temperature. The balance point had little to do with heating system optimization. On contrary, it was simply assumed that it was more cost efficient to run a single-circuit heat pump at all ambient conditions. In reality, the lower the gas price, the higher the ambient temperature at which it would be most efficient to switch to gas heat from running the electrically-powered heat pump. Therefore, the ambient temperature at which it becomes efficient to switch between gas and electricity is directly dependent on the ratio of gas-to-electric prices.
However, being a commodity, both electricity prices and gas prices fluctuate rapidly. It has been known, for example, that it is possible for electricity prices to vary by a factor of 10 or more during a 24-hour period, as electricity prices can be very low during off-peak hours at night and rise dramatically during a day. Gas prices can also fluctuate rapidly on a daily basis, though generally not as quickly as electricity prices.
Also, during the periods of high demand for electricity, the electric grid may operate close to a limit it can handle. Thus, along with rising cost for electricity during this time, there is a danger for the power supply interruptions and disruptions in the electric network operation that could lead to catastrophic consequences. Switching to gas or other supplemental heat in such circumstances will increase a safety margin of the electric grid operation.
Therefore, there is a need for a system and method that allows for adjustment of the switching set point between supplemental gas heating and electric operation of the heat pump system in real time, in response to changes in gas and electricity prices, to maximize savings to the consumer and to possibly prevent electric grid overloading.